TrkA specific signalling pathways are critical for mechanical allodynia development and bone alterations in a mouse model of rheumatoid arthritis

Role of TLR4 activation and signaling in bone remodeling, and afferent sprouting in serum transfer arthritis

BACKGROUND

In the murine K/BxN serum transfer rheumatoid arthritis (RA) model, tactile allodynia persists after resolution of inflammation in male and partially in female wild type (WT) mice, which is absent in Toll-like receptor (TLR)4 deficient animals. We assessed the role of TLR4 on allodynia, bone remodeling and afferent sprouting in this model of arthritis.

METHODS

K/BxN sera were injected into male and female mice with conditional or stable TLR4 deletion and controls. Paw swelling was scored and allodynia assessed by von Frey filaments. At day 28, synovial neural fibers were visualized with confocal microscopy and bone density assayed with microCT. Microglial activity and TLR4 dimerization in spinal cords were examined by immunofluorescence and flow cytometry.

RESULTS

In the synovium, K/BxN injected WT male and female mice showed robust increases in calcitonin gene related-peptide (CGRP+), tyrosine hydroxylase (TH)+ and GAP43+ nerve fibers. Trabecular bone density by microCT was significantly decreased in K/BxN WT female but not in WT male mice. The number of osteoclasts increased in both sexes of WT mice, but not in Tlr4-/- K/BxN mice. We used conditional strains with Cre drivers for monocytes/osteoclasts (lysozyme M), microglia (Tmem119 and Cx3CR1), astrocytes (GFAP) and sensory neurons (advillin) for Tlr4f/f disruption. All strains developed similar arthritis scores after K/BxN serum injection with the exception being the Tlr4Tmem119 mice which showed a reduction. Both sexes of Tlr4Lyz2, Tlr4Tmem119 and Tlr4Cx3cr1 mice displayed a partial reversal of the chronic pain phenotype but not in Tlr4Avil, and Tlr4Gfap mice. WT K/BxN male mice showed increases in spinal Iba1, but not GFAP, compared to Tlr4-/- male mice. To determine whether spinal TLR4 was indeed activated in the K/BxN mice, flow cytometry of lumbar spinal cords of WT K/BxN male mice was performed and revealed that TLR4 in microglia cells (CD11b+ /TMEM119+) demonstrated dimerization (e.g. activation) and a characteristic increase in lipid rafts.

CONCLUSION

These results demonstrated a complex chronic allodynia phenotype associated with TLR4 in microglia and monocytic cell lineages, and a parallel spinal TLR4 activation. However, TLR4 is dispensable for the development of peripheral nerve sprouting in this model.

CCL19-Positive Lymph Node Stromal Cells Govern the Onset of Inflammatory Arthritis via Tropomyosin Receptor Kinase

OBJECTIVE

The study objective was to assess the role of CCL19+ lymph node stromal cells of the joint-draining popliteal lymph node (pLN) for the development of arthritis.

METHODS

CCL19+ lymph node stromal cells were spatiotemporally depleted for five days in the pLN before the onset of collagen-induced arthritis (CIA) using Ccl19-Cre × iDTR mice. In addition, therapeutic treatment with recombinant CCL19-immunoglobulin G (IgG), locally injected in the footpad, was used to confirm the results. RNA sequencing of lymph node stromal cells combined with T cell coculture assays using tropomyosin receptor kinase (Trk) family inhibitors together with in vivo local pLN small interfering RNA (siRNA) treatments were used to elucidate the pathway by which CCL19+ lymph node stromal cells initiate the onset of arthritis.

RESULTS

Spatiotemporal depletion of CCL19+ lymph node stromal cells prevented disease onset in CIA mice. These inhibitory effects could be mimicked by local CCL19-IgG treatment. The messenger RNA sequencing analyses showed that CCL19+ lymph node stromal cells down-regulated the expression of the tropomyosin receptor kinase A (TrkA) just before disease onset. Blocking TrkA in lymph node stromal cells led to increased T cell proliferation in in vitro coculture assays. Similar effects were observed with the pan-Trk inhibitor larotrectinib in cocultures of lymph node stromal cells of patients with rheumatoid arthritis and T cells. Finally, local pLN treatment with TrkA inhibitor and TrkA siRNA led to exacerbated arthritis scores.

CONCLUSION

CCL19+ lymph node stromal cells are crucially involved in the development of inflammatory arthritis. Therefore, targeting of CCL19+ lymph node stromal cells via TRK could provide a tool to prevent arthritis.

Macrophages and glial cells: Innate immune drivers of inflammatory arthritic pain perception from peripheral joints to the central nervous system

Millions of people suffer from arthritis worldwide, consistently struggling with daily activities due to debilitating pain evoked by this disease. Perhaps the most intensively investigated type of inflammatory arthritis is rheumatoid arthritis (RA), where, despite considerable advances in research and clinical management, gaps regarding the neuroimmune interactions that guide inflammation and chronic pain in this disease remain to be clarified. The pain and inflammation associated with arthritis are not isolated to the joints, and inflammatory mechanisms induced by different immune and glial cells in other tissues may affect the development of chronic pain that results from the disease. This review aims to provide an overview of the state-of-the-art research on the roles that innate immune, and glial cells play in the onset and maintenance of arthritis-associated pain, reviewing nociceptive pathways from the joint through the dorsal root ganglion, spinal circuits, and different structures in the brain. We will focus on the cellular mechanisms related to neuroinflammation and pain, and treatments targeting these mechanisms from the periphery and the CNS. A comprehensive understanding of the role these cells play in peripheral inflammation and initiation of pain and the central pathways in the spinal cord and brain will facilitate identifying new targets and pathways to aide in developing therapeutic strategies to treat joint pain associated with RA.